The novel dimeric polyoxometalate [{beta-SiNi2W10O36(OH)(2)(H2O)}(2)](12-) (1) has been synthesized and characterized by IR spectroscopy, polarography, elemental analysis, thermogravimetric analysis, and magnetic measurements. An X-ray single-crystal analysis was carried out on K-12[{beta-SiNi2W10O36(OH)(2)(H2O)}(2)]. 20H(2)O, which crystallizes in the monoclinic system, space group P2(1)/n, with a = 13.701(4) Angstrom, b = 24.448(11) Angstrom, c = 13.995(5) Angstrom, beta = 99.62(3) degrees, and Z = 4. The anion consists of two [beta-SiNi2W10O36(OH)(2)(H2O)] Keggin moieties linked via two OH bridging groups, leading to a planar Ni-2(OH)(2) unit. The two half-units are related by an inversion center and each contain one Ni atom in the rotated triad. The formation of the new anion involves insertion, isomerization, and dimerization. Magnetic measurements show that the central Ni-4 unit exhibits ferromagnetic (J' = 4.14 cm(-1)) as well as weak antiferromagnetic (J = -0.65 cm(-1)) Ni-Ni exchange interactions.